A semiconductor device as described in the opening paragraph is known from a prior-art document, a European Patent Application published under no. EP 0 469 614 A1.
The known device comprises a semiconductor element formed by a substrate with an active front surface carrying an integrated circuit (or chip). This active surface also carries a plurality of contact pads for the input-output of the integrated circuit. This device in addition comprises a base plate formed by a substrate with a front surface which serves to support the semiconductor element and which for this purpose carries a plurality of contact pads. According to the cited document, this semiconductor element is turned over (or flipped) with its active surface towards the front surface of the base plate in order to connect the semiconductor element electrically to the base plate, and the patterns of the contact pads of the two mutually facing surfaces are provided so as to coincide. When the pads of the active surface of the flipped-over semiconductor element coincide with the pads of the front surface of the base plate, sealing means are provided so as to obtain the electrical connection between the respective contact pads of the semiconductor element and the base plate.
In the field of microwave ranges, it is necessary first of all to realise that all connections are transmission lines. The problem thus arises in a given circuit of how to integrate the active elements (transistors and diodes) and passive elements (resistors, capacitors, self-inductances, and microwave transmission lines) on one and the same chip. In this frequency range, on the other hand, it has proved necessary to integrate one or several purely passive circuits formed by transmission lines with the active circuit on the same chip, both for technical reasons and for economic reasons. These passive circuits have considerable dimensions. At 12 GHz, for example, such a circuit of the 90.degree. or 180.degree. phase shifter type will have transmission line elements of dimensions greater than one quarter of the length of the guided wave, which is thus 9 mm or 2.25 mm. The use of such passive elements in association with active elements thus leads to surface areas of integrated circuits which are not compatible with the surface areas of the substrates available in production, or which do not have a sufficiently low cost for obtaining industrially competitive devices.
The microwave circuits which comprise several sub-assemblies integrated on one and the same chip are called monolithic circuits (or MMICs=Monolithic Microwave Integrated Circuits). This monolithic integration is often difficult for various reasons as explained here.
In the microwave range, several other problems relating to the monolithic integration arise in addition to the surface problem indicated above.
One of these problems concerns the frequency limitation which arises when the transmission lines are subject to the influence of undesirable elements. These undesirable elements which disturb the propagation conditions for the millimeter-range waves in the lines may be formed in particular by the proximity of the plane of dielectric material of the front surface of the base plate upon the fixation of the semiconductor element on the latter, or alternatively by the proximity of the fixation material itself, owing to the fact that the circuit of lines is arranged so as to face this front surface; or alternatively by the proximity of other sub-assemblies of the monolithic circuit. The arrangement of transmission lines close to a dielectric which is alien to their structure, or to a radiating element, is to be avoided for all circuits of which the signals are very sensitive to dimensions and structures of the lines. This is particularly the case for the 90.degree. phase shifting circuits or 180.degree. phase shifting circuits mentioned above.
Another such problem relates to the frequency limitation which arises when there are discontinuities in the ground connections.
Another such problem relates to the appearance of parasitic capacitances when the connections between the microwave conductors are imperfect.